1. Field of the Invention
This invention relates in general to spin valve heads for magnetic storage systems, and more particularly to a method and apparatus for providing precise control of magnetic coupling field in NiMn top spin valve heads and amplitude enhancement.
2. Description of Related Art
Magnetic recording is a key and invaluable segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density or density of written data bits on the magnetic medium has increased by a factor of more than two million since the first disk drive was applied to data storage. Since 1991, areal density has grown by the well-known 60% compound growth rate, and this is based on corresponding improvements in heads, media, drive electronics, and mechanics.
Magnetic recording heads have been considered the most significant factor in areal-density growth. The ability of these components to both write and subsequently read magnetically recorded data from the medium at data densities well into the Gbits/in2 range gives hard disk drives the power to remain the dominant storage device for many years to come.
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly mounted on a slider that has an air-bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating. However, when the disk rotates, air is swirled by the rotating disk adjacent the ABS causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. The write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a sensing element, fabricated of a magnetic material, as a function of the strength and direction of magnetic flux being sensed by the sensing element. Conventional MR sensors, such as those used as a MR read heads for reading data in magnetic recording disk drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy (Ni81Fe19). A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the disk in a disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element and a corresponding change in the sensed current or voltage.
In the past several years, prospects of even more rapid performance improvements have been made possible by the discovery and development of sensors based on the giant magnetoresistance (GMR) effect, also known as the spin-valve effect. Magnetic sensors utilizing the GMR effect, frequently referred to as “spin valve” sensors, typically involve a sandwiched structure consisting of two ferromagnetic layers separated by a thin non-ferromagnetic layer. One of the ferromagnetic layers is called the “pinned layer” because it is magnetically pinned or oriented in a fixed and unchanging direction by an adjacent anti-ferromagnetic layer, commonly referred to as the “pinning layer,” through anti-ferromagnetic exchange coupling. The other ferromagnetic layer is called the “free” or “unpinned” layer because the magnetization is allowed to rotate in response to the presence of external magnetic fields.
The benefits of spin valve sensors result from the large change of conductance exhibited by the devices, which depends on the relative alignment between the magnetizations of the two ferromagnetic layers. In order to function effectively, a sufficient pinning field from the pinning layer is required to keep the pinned ferromagnetic layer's magnetization unchanged during operation of the GMR sensor. Thus far, various anti-ferromagnetic materials, such as FeMn, NiO, IrMn, PtPdMn, and TbCo, have been used as pinning layers for spin valve sensors. However, these materials have provided less than desirable results. For example, for FeMn pinning layers, the temperature (referred to as the blocking temperature) at which the pinning field disappears (or is greatly reduced) is very close to the typical sensor operating temperature of 100° C.–150° C. Therefore, at normal operating temperatures, an FeMn pinning layer typically does not provide a pinning field of sufficient strength to prevent the magnetization of the pinned ferromagnetic layer from rotating in the presence of an external magnetic field. Without sufficient pinning strength, the spin valve cannot function to its full potential. Further, materials such as FeMn and TbCo are susceptible to corrosion. Also, oxide materials such as NiO, which provide a low pinning field as well, are difficult to process. IrMn and PtPdMn are both expensive materials which provide pinning field strengths which are lower than is desired at normal operating temperatures.
NiMn has properties which make it desirable for use as an exchange bias layer material to stabilize magnetic sensors. See for example, Lin et al., “Improved Exchange Coupling Between Ferromagnetic Ni—Fe and Antiferromagnetic Ni—Mn-based Films,” Appl. Phys. Lett., Vol. 65, No. 9, pp. 1183–1185, Aug. 29, 1994. See also, Lin et al., U.S. Pat. No. 5,315,468 entitled “Magnetoresistive Sensor Having Antiferromagnetic Layer for Exchange Bias.” These references discuss the use of NiMn as an exchange bias layer material to stabilize the MR sensor layer. However, the exchange fields must be kept low to avoid pinning the sensor layer, which would drastically reduce its sensitivity. NiMn is capable of providing high exchange fields at temperatures far in excess of the pinning layer materials mentioned above. In addition to its ability to provide thermally stable exchange fields of high magnitude, NiMn is very corrosion-resistant. As taught by Lin et al., a NiMn layer having a thickness of around 500 Å can be used as an exchange bias layer adjacent to the MR sensor layer in a conventional MR sensor. However, the thickness and structure required for the MR sensor of Lin et al. are not compatible with necessary spin valve sensor thickness and structures.
A problem with using NiMn as a pinning layer material in a spin valve sensor is that, as is well-known in the art, heating a spin valve sensor to temperatures greater than 225°–240° C. for more than 2–3 hours has resulted in inter-diffusion of the various layers and thus in destruction of the sensor. However, high annealing temperatures are necessary in order to realize the high pinning fields desired from the NiMn. The inter-diffusion between the layers during high temperature annealing has been an obstacle to using NiMn as a pinning layer in spin valve sensors. See for example, Devasahayam et al., “Exchange Biasing with NiMn,” DSSC Spring '96 Review, Carnegie Mellon University.
Attempts in the prior art to create a spin valve sensor having NiMn as a pinning layer have failed. For example, Devasahayam et al. describe one such failed attempt in which the NiMn pinning layer is pre-annealed prior to depositing the NiFe ferromagnetic layer of the spin valve sensor. Devasahayam et al. describe another attempt to use NiMn as a pinning layer material in a spin valve sensor in which a bi-layer of NiMn and NiFe are pre-annealed. Next, the layer of NiFe is sputter etched away, and a new NiFe ferromagnetic layer is deposited on top of the NiMn pinning layer. While some success was reported in this second attempted method, the device reported by Devasahayam et al. requires a NiFe ferromagnetic layer thickness of 250 Å and a NiMn pinning layer thickness of 500 Å, while achieving a pinning field of only 100 Oe.
Thus, in addition to providing insufficient pinning strengths, the thicknesses of the layers required by Devasahayam et al. are incompatible with spin valve sensor requirements. Further, the process of annealing and sputter etching the layer of NiFe and redepositing the layer of NiFe is not practical for use in producing spin valve sensors. Therefore, there is a need for a spin valve sensor with thermally stable high pinning fields.
Spin valve sensors have been developed that include a first layer of ferromagnetic material and a second layer of ferromagnetic material, with the second layer of ferromagnetic material having a thickness of less than about 100 Å. A first layer of non-ferromagnetic conducting material is positioned between the first and second layers of ferromagnetic material. A NiMn pinning layer is positioned adjacent to the second layer of ferromagnetic material such that the pinning layer is in contact with the second layer of ferromagnetic material, wherein the NiMn pinning layer has a thickness of less than about 200 Å and provides a pinning field for pinning a magnetization of the second layer of ferromagnetic material in a first direction.
However, In such NiMn top spin valve heads, the Cu seed layer prior to deposition of magnetic free layers plays an important role in affecting the magnetic coupling field and amplitude. With increasing Cu seed layer thickness, the ferromagnetic coupling field decreases sharply and stays constant at 13 Å of Cu thickness, while the GMR effect increases up to 20 Å of Cu thickness due to spin filtering effect. Typically the ferromagnetic coupling field of NiMn spin valve heads with 15 Å thick Cu seed layer is 8–10 Oe and is difficult to be adjusted unless the Cu spacer thickness is changed. The precise control of magnetic coupling field is important in yielding high performance heads, since the coupling field sensitively affects the head performance such as asymmetry, amplitude and asymmetry uniformity within the wafer. Besides, it is difficult to fabricate the head having a negative coupling field using a NiMn spin valve structure because of interdiffusion of free and pinned layers during high temperature annealing process (e.g., ˜250° C.)
It can be seen that there is a need for a method and apparatus for providing precise control of magnetic coupling field in NiMn top spin valve heads and amplitude enhancement.